The background of the invention will be presented by reference to (A) our practical knowledge of the art and the problems to be overcome, and (B) a study of the prior art literature which we carried out pursuant to, and to check the novelty of, the invention which we have developed to meet such problems. It is against such background that the contribution which the invention makes to the art should be viewed.
(A) Asbestos cement sheets, either flat or corrugated, have a wide present day usage in the building industry in providing durable, reasonably strong and workable, water proof wall and roof claddings having a high dimensional stability; thicker stronger sheets or boards of such material are also usable as the bases of floors in areas such as bathrooms and laundries. The sheets find a particular usage in regions where insect attack would soon destroy timber house claddings such as weatherboards. The durability of asbestos cement in microbial degrading conditions also leads to its use in the manufacture of articles such as prefabricated septic tanks, plant troughs, ground gutters, and to the use of corrugated sheets as fence panels.
Unfortunately it has been recently discovered that asbestos fibres are dangerous to human health--and that people working in an environment involving air suspension of such fibres may subsequently develop lung cancer and other respiratory diseases. The dangers appear such that many countries of the world have either introduced, or are considering the introduction of, a variety of restrictions on the use of asbestos fibres. As medical evidence of the dangers accumulates, it is possible that all concerned countries may either prohibit the mining or use of asbestos fibres or may require safeguards, the implementation of which would substantially increase the cost of manufacturing and using asbestos cement fibre sheets.
In view of the above dangers, and/or potential cost increases, there is clearly a need for a building sheet that does not include asbestos fibres but that does, at the same time, have similar properties to an asbestos cement sheet such as: comparable cost, low water permeability, density, strength, flexibility, durability, fire resistance, surface smoothness, handleability, workability and in-service characteristics (re the last three properties see immediately below). Briefly, our invention, as hereafter defined and described, meets such need.
Note: The term "handleability" refers to the ability of a sheet to be carried and fixed without breaking--and is usually related to measurable characteristics such as: modulus of rupture, modulus of elasticity, hardness, impact strength and apparent specific gravity (these properties are defined in the ensuing description). The term "workability" refers to the ability of a sheet to be cut, nailed, drilled and holed. Finally, the expression "in-service characteristics" refers to durability and the ability of a sheet to stand up to the stresses that it will experience during its lifetime after installation; such stresses include those resulting from hygro-expansion and freeze thaw conditions. It is important that a building sheet have these properties.
In seeking a solution to the problems associated with the use of asbestos fibres, one approach has been to replace such fibres with glass fibres. However, this approach on the whole has not been successful, because of the cost of the glass fibres and the lack of durability resulting from the solubility of glass, over a period of time, in a cement environment (throughout this specification, "cement" means a Portland-type cement). Whilst solubility can be countered in part by coating the fibres, this in turn introduces other problems as well as leading to further cost increases. Attempts to manufacture "special" glass formulations, having a low solubility, have also failed because of cost considerations. A further difficulty has been that glass/cement slurries have filtering characteristics that make them difficult to form on standard asbestos cement forming machines.
Another approach has been to replace the asbestos fibres with cellulose fibres. However, this approach appeared regressive since asbestos fibres, when first introduced, overcame strength and durability problems associated with early prior use of cellulose fibres for reinforcing cements of the Portland type. These prior problems will now be discussed briefly since, being associated with the use of cellulose fibres, they are among problems which one might not have expected to be overcome by the present invention.
The strength problem is briefly as follows: lignin, and other impurities, associated with cellulose fibres interfere with the setting of the cement. Cement, when set, does not seem to have a good bond with the surfaces of cellulose fibres--and thus, inordinate movement can take place between the fibres and the set cement. In addition, difficulties have even been encountered in the mixing of a cement containing cellulose fibres.
Durability problems typically arise because cellulose is an organic material and is subject to microbial degradation--and possible termite attack. The problem of microbial degradation can be especially dangerous if the article manufactured from the fibre reinforced cement has a structural use since the article may suddenly fail at some unexpected date after installation. Such a sudden failure usually results because of property changes, such as loss of alkalinity, that continuously takes place in the set cement, such changes producing a more favourable environment for microbes (the problem of termite attack on cellulose is of course well known). Hygro-expansion is also a problem. A portland cement matrix is itself hygro-expansive as are cellulose fibres--which means that the quantity of cellulose which may be incorporated in the cement matrix is limited by consideration of the hygro-expansion.
The above prior problems--which in the past were overcome by replacing cellulose fibres by asbestos fibres--have not been reintroduced by the present invention because, in our invention, we overcome the problems by replacing cement by a suitable oxide or hydroxide and by replacing the water setting process by a carbonation hardening process whereby the oxide or hydroxide is converted into a carbonate mass. In addition, the present invention is predicated on our surprising discovery that if one forms the slurry (of cellulose fibres, water, and selected oxides or hydroxides) so as to have a porosity and cellulose content within specified ranges, the product of the subsequent carbonation step will be a fibre reinforced building element which (as will be seen hereafter) is a completely satisfactory replacement for the asbestos cement sheets of the prior art.
(B) Our discovery, as indicated above, led us to study the prior art literature--which study revealed several publications that fall into the category of related background material.
None of these documents, however, describes a method as defined hereafter, and in the appended claims. In particular, none of the methods described in these documents produces an article or shape having an uncarbonated porosity and cellulose content within the ranges laid down in this specification.